Method and apparatus for calculating image correction data and projection system

ABSTRACT

An image correction data calculation method comprises the steps of acquiring an input-output characteristic at each of a plurality of display elements on a display screen of an image display device, on the basis of image data captured by a CCD camera (steps S 101  to S 105 ), setting a target input-output characteristic to be obtained at each of the plurality of display elements (step S 106 ), and calculating image correction data used to correct the input-output characteristic for an input image signal, according to the locations of display elements on the screen, on the basis of the input-output characteristic acquired in the acquisition step and the target input-output characteristic to be obtained (step S 107 ).

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/JP03/01639 filed Feb. 17, 2003.

TECHNICAL FIELD

The present invention relates to a method and apparatus for calculatingimage correction data for correcting nonuniformity of luminance and/orcolor in a projection system using a single image display device or in amultiprojection system in which a single screen is formed using aplurality of image display devices. The present invention also relatesto a projection system.

BACKGROUND ART

In general, a multiprojection system includes a screen on which toproject an image, a plurality of projectors for projecting images torespective assigned areas on the screen, and a projector arraycontroller for supplying, to each projector, a video signal associatedwith an image to be projected by each projector.

A seamless projection system is known as one type of multiprojectionsystem. In the seamless projection system, a plurality of images areprojected by a plurality of projectors onto a screen such that theimages are partially overlapped on the screen, thereby forming a singleseamless image. To obtain a single image on a screen in such a seamlessprojection system, it is necessary to precisely align individual imagesprojected on the screen such that edge lines of adjacent individualimages precisely coincide with each other.

In the multiprojection system, respective projectors are different inluminance and color, and thus correction in terms of luminance and coloris necessary.

A known method or apparatus for calculating image correction data usedto correct luminance difference or color difference may be found, forexample, in Japanese Unexamined Patent Application Publication No.7-226862 (first conventional technique) or Japanese Unexamined PatentApplication Publication No. 5-173523 (second conventional technique).

In the first conventional technique, an output signal of an image signalassociated with an image to be projected is compared with a referencesignal prepared in advance. Image correction data is produced on thebasis of a difference detected in the comparison. The image signal iscorrected by applying the produced image correction data to the imagesignal, and the resultant output signal is compared with the referencesignal. The above process is performed repeatedly until optimum imagecorrection data, which allows the output signal to be substantiallyequal to the reference signal, is obtained.

In the second conventional technique, an image signal with a uniform andconstant level is input to respective projectors of a multiprojectionsystem, and, first, an adjustment is made such that the output signallevel of each projector becomes uniform. Then, a further adjustment ismade such that the output signal levels of the respective projectorsbecome equal to each other. Final adjusted values used in the aboveadjustment are employed as image correction data.

Any projector has more or less nonuniformity of luminance because theinput-output characteristic representing the relationship between thelevel of an input image signal and the corresponding level of outputluminance signal varies depending on the display position (hereinafter,the input-output characteristic will be referred to as a gammacharacteristic). In the case of a color projector, more or lessnonuniformity of color occurs owing to differences in gammacharacteristic among primary colors.

In particular, in a multiprojection system, because one screen is formedusing a plurality of projectors, differences in gamma characteristicamong projectors can cause rather large nonuniformity in luminanceand/or color. In a case of a seamless multiprojection system, the gammacharacteristic in overlapping areas is influenced by a plurality ofprojectors.

In the method of producing shading correction data according to thefirst conventional technique, correction data is produced such thatnonuniformity in luminance and color is eliminated at a signal level ofan image to be displayed. Therefore, although nonuniformity in luminanceand color is corrected for an input image with a particular signallevel, nonuniformity in luminance or color is not necessarily eliminatedfor input images with other different signal levels.

In the second conventional technique, image correction data used tocorrect luminance nonuniformity in a multiprojection system is producedas follows. First, an adjustment is made for each projector such thatluminance nonuniformity is eliminated, and then a further adjustment ismade such that luminance differences among projectors are minimized. Theadjustment data finally used in the adjustment is employed as the imagecorrection data. In this method, a large number of steps are needed toproduce the image correction data. Besides, it is not possiblepractically to acquire image correction data for each display position.When projectors have overlap areas on a screen as in seamlessmultiprojection systems, each overlap area is influenced bycorresponding two projectors. This makes it very difficult to obtainaccurate image-correct data because it is necessary to simultaneouslymake an adjustment of luminance uniformity and color uniformity withineach projector and also among the projectors.

In view of the above, it is an object of the present invention toprovide a method and apparatus for calculating image correction data,which allow automatic and easy production of image correction data foreach display position of a projector, for use in reducing at least oneof luminance nonuniformity and color nonuniformity.

It is another object of the present invention to provide a projectionsystem capable of projecting a seamless image whose luminancenonuniformity and/or color nonuniformity are reduced using producedimage correction data, for all input image signal levels.

DISCLOSURE OF INVENTION

The present invention provides a method of calculating image correctiondata, comprising a step of acquiring an input-output characteristic ateach of a plurality of display elements on a display screen of an imagedisplay device including one or more image display units, on the basisof image data captured by image capture means, a step of setting atarget input-output characteristic to be obtained at each of theplurality of display elements, and a step of calculating imagecorrection data used to correct the input-output characteristic for aninput image signal, depending on the locations of display elements onthe screen, on the basis of the input-output characteristic acquired inthe acquisition step and the target input-output characteristic to beobtained.

The present invention also provides an image correction data calculatorfor calculating image correction data on the basis of a measuredinput-output characteristic of an image display device, wherein theimage correction data calculator comprising: characteristic measurementmeans for acquiring an input-output characteristic at each of aplurality of display elements on a display screen of an image displaydevice, on the basis of image data captured by image capture means;image correction means for setting a target input-output characteristicto be obtained at each of the plurality of display elements; andcalculation means for calculating image correction data on the basis ofthe input-output characteristic measured by the characteristicmeasurement means and the target input-output characteristic to beobtained.

The present invention also provides a projection system having acapability of correcting an image using image correction data, whereinthe projection system comprising: image output means for outputtingimage data to be displayed; image correction means for correcting theimage data output from the image output means in accordance with theimage correction data; and image display means for displaying the imagedata corrected by the image correction means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B are a block diagram showing a projection systemincluding an image correction data calculator and a processing unit,according to a first embodiment of the present invention;

FIG. 2 is a flow chart showing an image correction data calculationmethod implemented by the image correction data calculator according tothe first embodiment of the present invention;

FIG. 3 is a characteristic diagram showing measured luminancedistributions along display positions, in the image correction datacalculator according to the first embodiment of the invention;

FIG. 4 is a characteristic diagram showing target luminancedistributions to be obtained along display positions, in the imagecorrection data calculator according to the first embodiment of theinvention;

FIG. 5 is a characteristic diagram, which are smoothly varying along aline of the display positions, showing target luminance distributions tobe obtained in the image correction data calculator according to thefirst embodiment of the invention, wherein the luminance distributionsare obtained by passing a captured image signal through a lowpassfilter;

FIG. 6 is a characteristic diagram showing predetermined targetluminance distributions along a line of display positions, in the imagecorrection data calculator according to the first embodiment of theinvention;

FIG. 7 is a characteristic diagram showing a method of calculating gammacorrection data on the basis of a measured gamma characteristic and atarget gamma characteristic, in the image correction data calculatoraccording to the first embodiment of the invention;

FIG. 8 is a flow chart showing an example of a process performed by theimage correction data calculator to calculate gamma correction data usedto correct an image including a plurality of color components, accordingto the first embodiment of the invention;

FIG. 9 is a block diagram of an image correction data calculatoraccording to a second embodiment of the present invention;

FIG. 10 is a flow chart showing an image correction data calculationmethod implemented by the image correction data calculator according tothe second embodiment of the present invention;

FIG. 11 is a flow chart showing a process of acquiring measured gammacharacteristics using a filter properly selected for each of RGB colors,in the image correction data calculator according to the secondembodiment of the present invention;

FIG. 12 is a flow chart showing a process of acquiring white balancedata and measured gamma characteristics by using white balanceadjustment filters, in the image correction data calculator according tothe second embodiment of the present invention;

FIG. 13 is a flow chart showing a process of producing optimum gammacorrection data according to a third embodiment of an image correctiondata calculation method of the present invention;

FIG. 14 is a flow chart showing another example of a process ofacquiring measured gamma characteristics based on the image correctiondata calculation method according to the third embodiment of the presentinvention;

FIG. 15 is a block diagram showing an image correction unit used in aprojection system according to a fourth embodiment of the presentinvention;

FIG. 16 is a block diagram showing an image correction unit used in aprojection system according to a fifth embodiment of the presentinvention;

FIG. 17 is a block diagram showing an image correction unit used in aprojection system according to a sixth embodiment of the presentinvention;

FIG. 18 is a block diagram showing an arrangement of lookup tables LUT1to LUTn in an image correction unit of a projection system according tothe sixth embodiment of the present invention.

FIG. 19 is a block diagram showing an image correction unit used in aprojection system according to a seventh embodiment of the presentinvention;

FIG. 20 is a flow chart showing a process of calculating data to bestored in LUTs disposed in the image correction unit of the projectionsystem according to the seventh embodiment of the invention;

FIG. 21 is a block diagram showing a first modification of the imagecorrection unit used in a projection system, according to the seventhembodiment of the present invention; and

FIG. 22 is a block diagram showing a second modification of the imagecorrection unit used in a projection system, according to the seventhembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described below with referenceto the accompanying drawings.

(First Embodiment)

FIGS. 1A to 7 are diagrams explaining a first embodiment of the presentinvention. FIG. 1A and FIG. 1B are a block diagram showing a projectionsystem including an image correction data calculator and a processingunit, according to a first embodiment of the present invention.

As shown in FIG. 1A, in this first embodiment of the present invention,an image correction data calculator 1 mainly includes: a personalcomputer (PC) 3 serving as processing means for executing an operatingsystem and an image correction data calculation program to implement animage correction data calculation method according to the firstembodiment of the invention and also serving as characteristicmeasurement means; an image correction unit 5 serving as imagecorrection means for correcting an image signal output from the PC 3 inaccordance with image correction data; a liquid crystal projector(referred to simply as a “projector” in the drawings and hereinafter inthe present description) 7 serving as an image display device and imagedisplay means for projecting an image in accordance with image dataoutput from the image correction unit 5 and also serving ascharacteristic measurement means; a screen 8 serving as an image displaydevice and image display means for displaying the image projected by theliquid crystal projector 7; and a CCD (Charge Coupled Device) camera 9serving as image capture means for capturing an image displayed on thescreen 8 and also serving as characteristic measurement means.

The PC 3 includes, at least, an image input board 32 for inputting, tothe PC 3, a digital image signal output from the CCD camera 9, aprocessing unit 30 serving as analysis means for calculating imagecorrection data on the basis of the image signal input via the imageinput board 32 by executing the image correction data calculationprogram and also serving to execute the operating system and otherprocessing, a correction data transfer board 33 for transferring gammacorrection data, which is image correction data calculated by theprocessing unit 30, to the image correction unit 5, and a graphic board31 serving as image output means for outputting, under the control ofthe processing unit 30, an analog signal including R (Red), G (Green)and B (Blue) components in a VGA (Video Graphics Array) format to theimage correction unit 5.

More specifically, as shown in FIG. 1B, the processing unit 30 includesan input-output characteristic acquisition unit 30 a for acquiring aninput-output characteristic on the basis of the captured image input viathe image input board 32, a target input-output characteristiccalculation unit 30 b for determining a target input-outputcharacteristic by means of calculation based on the captured image inputvia the image input board 32, and a correction data calculation unit 30c for calculating correction data based on the input-outputcharacteristic acquired via the input-output characteristic acquisitionunit 30 a and the target input-output characteristic calculated by thetarget input-output characteristic calculation unit 30 b.

The projection system 11 is formed by the PC 3, the image correctionunit 5, the liquid crystal projector 7, and the screen 8.

The operation of image correction data calculator constructed in theabove-described manner according to the first embodiment is describedbelow based on FIG. 1A and FIG. 1B and with reference to FIGS. 2 to 7.FIG. 2 is a flow chart showing an image correction data calculationmethod implemented by the image correction data calculator according tothe first embodiment of the present invention.

The image correction data calculator 1 produces gamma correction data,that is, image correction data, by performing a process including threemain steps (S10, S20, and S30) shown in FIG. 2.

In step S10 (acquisition of measured gamma characteristic), the gammacharacteristic corresponding to a display position of the image displaydevice formed by the liquid crystal projector 7 and the screen 8 isacquired (the display position corresponds to a display element (that isa display element (generally called a display pixel) as the basic unitin displaying an image, and thus the display position will also bereferred to as a display element. Accordingly, the notation such as“display element” or “display pixel” is used also in the drawings).

In step S20 (calculation of target gamma characteristic), a target gammacharacteristic corresponding to the measured display element is set.

In step S30, gamma correction data is calculated by the PC 3 serving asthe processing means.

The acquisition of the measured gamma characteristic in step S10 isdescribed in further detail below.

The gamma characteristic is synonymous with the input-outputcharacteristic of the system. Thus, the gamma characteristic can beacquired by measuring an input signal and an output signal correspondingto the input signal.

In the present embodiment, the input signal is defined as a signalapplied to the graphic board 31 from the processing unit 30 of the PC 3.A signal output from the graphic board 31 is projected onto the screen 8by the liquid crystal projector 7, and an image displayed on the screen8 is captured by the CCD camera 9. An image signal output from the CCDcamera 9 is defined as the output signal described above.

Herein, by way of example, it is assumed that the input signal takes an8-bit value (from “0” to “255”) indicating a gray level, and the outputsignal also takes an 8-bit value (from “0” to “255”) (this assumption isused in many actual projection systems).

In order to acquire gamma characteristic data in a perfect form, it isneeded, theoretically, to acquire the output signal level correspondingto each of all allowable input signal levels. In practice, anapproximated gamma characteristic data can be acquired by acquiringinput-output data for levels taken at particular intervals bit values inthe dynamic range from “0” to “255” and calculating input-output data atthe other levels by means of interpolation. The input signal levelsselected for use in acquisition of the approximated gamma characteristicdata are referred to as sampling points.

In this first embodiment, input signal levels of “0”, “16”, “32”, “48”,“64”, “80”, “96”, “112”, “128”, “144”, “160”, “176”, “192”, “208”,“224”, “240”, and “255” are selected as measurement sampling points, anda test image having a signal level corresponding to one of themeasurement sampling points for predetermined display elements is storedin the PC 3 in advance for all the sampling points. More specifically,for example, for a sampling point “0”, the test image has a signal levelof “0” at predetermined ones of 2-dimensionally arranged displayelements of the image display device. For a sampling point “16”, thetest image has a signal level of “16” at the predetermined displayelements. Test image data may include a signal with a levelcorresponding to each sampling point to all display elements to allowacquisition of gamma characteristic data for all display elements,although a longer processing time is needed.

To acquire the measured gamma characteristic, first, initial gammacorrection data which has no actual correction effect in this initialstate is transferred from the PC 3 to the image correction unit 5 viathe correction data transfer board 33.

Thereafter, the processing unit 30 of the PC 3 executes the imagecorrection data calculation program according to the first embodiment ofthe invention thereby outputting a test image with a signal levelcorresponding to a sampling point “0” to the image correction unit 5 viathe graphic board 31. Because the gamma correction data has nocorrection capability at this stage, the image correction unit 5performs gamma correction on the received test image data using standardgamma correction data and outputs the resultant data to the liquidcrystal projector 7. The liquid crystal projector 7 projects an image inaccordance with the received data thereby displaying the test image onthe screen 8 (step S101).

Thereafter, the processing unit 30 of the PC 3 controls the CCD camera 9to capture a whole image displayed on the screen 8. The image datacaptured by the CCD camera 9 is stored in a storage device of the PC 3via the image input board 32 (step S102).

In the above process of taking the image using the CCD camera 9, theimage can be more accurately captured and measurement accuracy can beimproved by blocking external light in an environment where the image istaken, automatically adjusting exposure of the CCD camera 9, and/orcumulatively adding the signal output from the CCD camera 9 in capturingthe image.

In the next step (S103), the processing unit 30 of the PC 3 determineswhether the processing on the test image is completed for all samplingpoints. If the processing is not completed, steps S101 to S103 arerepeated, changing the sampling point, until the processing on the testimage is completed for all sampling points.

When it is determined in step S103 that the processing unit 30 of the PC3 has captured the image for all sampling points, the process ofdetermining the gamma characteristic of the predetermined displayelements from the captured image is started.

The processing unit 30 of the PC 3 acquires an output signal at asampling point, on the basis of the correspondence between the locationof the predetermined display elements and the pixel location of the CCDcamera 9. An output signal for an input signal with a level other thanthe sampling points can be calculated by means of interpolation fromoutput signals obtained for input signals with levels close to samplingpoints. In the interpolation for the above purpose, linear interpolationor spline interpolation may be employed. The processing unit 30 of thePC 3 performs the above-described process for all of the predetermineddisplay elements, and calculates the measured gamma characteristic curvefor each of all the predetermined display elements from the acquiredinformation (step S104).

The processing unit 30 of the PC 3 calculates the gamma characteristicof each of display elements other than the predetermined displayelements by means of interpolation or extrapolation from the gammacharacteristics of the predetermined display elements (step S105). Whenit is desirable to perform the calculation in a short time or when theavailable storage capacity is limited, the display screen may be dividedinto blocks each including, for example, 4×4 display elements, and thegamma characteristic may be determined for each block on the assumptionthat all display elements in the same block are substantially equal ingamma characteristic. The block size may be adaptively varied dependingon the luminance nonuniformity. That is, the block size is set to besmall in an area having large luminance nonuniformity, and the blocksize is set to be large in an area having small luminance nonuniformity.The data size of the gamma characteristic data may be reduced byanalyzing the gamma characteristic data obtained for each of all displayelements, grouping together similar gamma characteristics, andexpressing the gamma characteristic of each group by a singlerepresentative gamma characteristic.

Thus, the gamma characteristic at each display position on the displayscreen is obtained by performing the above-described process.

Now, the process of setting (calculating) a target gamma characteristicto be obtained in main step S20 on the basis of the acquired information(in step S106) is described.

In this image correction data calculator 1 according to the firstembodiment of the present invention, the CCD camera 9 takes the place ofa human eye. That is, when there is no irregularity in the distributionof luminance over the image captured by the CCD camera 9, the imagedisplay device is regarded as having no irregularity in the distributionof luminance. When the luminance changes smoothly, the image displaydevice can also be regarded as having no irregularity in thedistribution of luminance. A target image corresponding to the capturedimage at the sampling point is set, and the target gamma characteristiccan be calculated by applying to the target image the same calculationmethod used to obtain the gamma characteristic at each display elementof the display screen.

The method of setting a target image for a given sampling point isdescribed in detail below.

Referring to FIG. 3, an example of a measured luminance distributionwill be described. FIG. 3 is a characteristic diagram obtained, in theimage correction data calculator according to the first embodiment ofthe invention, by plotting the luminance distribution along apredetermined display element line (generally called a pixel line) in a2-dimensional image of the screen 8 for each input signal level. In FIG.3, the horizontal axis represents the element position, and the verticalaxis represents the luminance.

Note that FIG. 3 shows luminance distribution line of sampling pointsfor respective input signal levels of “0”, “n”, and “255”. As can beseen, the luminance varies depending on the element position even for anequal input signal level.

With reference to FIG. 4, a first method of setting the target image isdescribed below. FIG. 4 is a characteristic diagram plotting a targetluminance distribution to be obtained along the display element line inthe image correction data calculator according to the first embodimentof the invention. In FIG. 4, the horizontal axis represents the elementposition, and the vertical axis represents the luminance.

In this first method of setting the target image, the target image isset such that, as shown in FIG. 4, the target image has a uniformluminance distribution, that is, equal luminance is obtained at anydisplay element, for a maximum input signal level (“255”), and for aminimum input signal level (“0”), the target image also has a uniformluminance distribution, that is, the luminance becomes equal at anydisplay element, and furthermore, for an input signal level between theminimum and maximum signal levels, the luminance at any display elementbecomes equal to the average of luminance values of all pixels of thedisplay screen image captured by the camera.

With reference to FIG. 5, a second method of setting the target image isdescribed below. FIG. 5 is a characteristic diagram showing smoothlyvarying luminance distributions to be obtained in the target image inthe image correction data calculator according to the first embodimentof the invention, wherein the luminance distributions are obtained bypassing the captured image signal through a lowpass filter. In FIG. 5,the horizontal axis represents the element position, and the verticalaxis represents the luminance.

In this second method of setting the target image, as shown in FIG. 5, asmoothly varying luminance distribution is obtained by passing thecaptured image signal through the lowpass filter for each of maximum,minimum, and intermediate input signal levels. More specifically, thetarget image for the input signal with the maximum level (“255”) isobtained by passing through the lowpass filter the image signal capturedfor the input signal with the level of “255”. The target image for theinput signal with the minimum level (“0”) is obtained by passing throughthe lowpass filter the image signal captured for the input signal withthe level of “0”. On the other hand, for an input signal with anintermediate level between the maximum and minimum levels, the gammacharacteristic curve is set for the output luminance at each pixel suchthat equation (1) described below is satisfied.(output luminance)=(input signal value)^(γ)  (1)wherein γ is a constant.

This allows the calculation of the target gamma characteristic to besimplified, and thus the calculation can be performed in a short time.Instead of using equation (1) in the calculation, a table representingan input-output characteristic may be used.

The second method of setting the target image allows for the targetgamma characteristic to obtain higher contrast than the contrastobtained by the first method of setting the target image.

With reference to FIG. 6, a third method of setting the target image isdescribed below. FIG. 6 is a characteristic diagram showing luminancedistributions along the scanning line of the target image obtained bythe image correction data calculator according to the first embodimentof the invention. In FIG. 6, the horizontal axis represents the elementposition, and the vertical axis represents the measured luminance.

In this third method of setting the target image, the target image forthe input signal with the maximum level (“255”) is set so as to have apredetermined luminance distribution. Similarly, the target image forthe input signal with the minimum level (“0”) is set so as to have aseparately predetermined luminance distribution. For input signals withlevels between the minimum and maximum levels, the luminancedistribution of the target image is calculated in accordance with anequation or a table.

In this third method of setting the target image, the target image canbe set so as to have a predetermined luminance distribution, that is, itis possible to achieve a desired distribution. For example, the targetimage can be set so as have high contrast only in a central area of thescreen.

After the target image is set for all sampling points in theabove-described manner, corresponding target gamma characteristics arecalculated in a similar manner as in the process of calculating thegamma characteristic.

A method of calculating the gamma correction data in main step S30 onthe basis of the measured gamma characteristic and the target gammacharacteristic (in step S107) is described below with reference to FIG.7.

FIG. 7 is a characteristic diagram showing a method of calculating gammacorrection data on the basis of a measured gamma characteristic and atarget gamma characteristic, in the image correction data calculatoraccording to the first embodiment of the invention. In FIG. 7, thehorizontal axis represents the input signal, and the vertical axisrepresents the signal output from the CCD camera 9.

In FIG. 7, a measured gamma characteristic (measured gamma curve) Go ofan arbitrary display element i on the display screen and a correspondingtarget gamma characteristic (target gamma curve) Gt are plotted.

Gamma correction data is obtained by producing a lookup table for aninput signal such that application of the gamma correction data causesthe measured gamma curve Go to be converted to be equal to the targetgamma curve Gt.

That is, as can be seen from FIG. 7, the measured gamma curve Goindicates that when an input signal s is applied to the display elementi, an output signal u is obtained. On the other hand, the target gammacurve Gt indicates that the target output value corresponding to theinput signal s is v.

To obtain the target output of v in the measured gamma curve Go, theinput signal must be t. In other words, when the input signal is s, andthis input signal is converted into t, then the measured gammacharacteristic becomes equivalent to the target gamma characteristic.The conversion from the signal s to the signal t can be achieved bygamma correction. By determining the converted value t corresponding tothe input signal s over the entire dynamic range (from “0” to “255” inthis specific example), gamma correction data for the display element ican be obtained. This determination process is performed by theprocessing unit 30 of the PC 3.

In the calculation, the converted value t corresponding to the inputsignal s can be negative or can be greater than “255”. However, in thepresent example, the gamma correction data is produced by the PC 3 onthe assumption that input and output signals are both within the rangethat can be represented by 8 bits (that is, from “0” to “255”), and thusvalues out of the above range are clipped between “0” to “255” such thatany value falls within the range.

Thus, the gamma correction data for each display position of the screenis obtained. By calculating the gamma correction data for each displayelement of the image display device in the above-described manner, itbecomes possible to make a correction such that a uniform luminancedistribution is obtained over the screen.

If the display screen is divided into a plurality of blocks eachincluding a plurality of display elements, and the image correction datais determined for each block, thus the image correction data can becalculated in a short time.

A process of correcting image data in accordance with image correctiondata determined by the image correction data calculator using the imagecorrection data calculation method according to the first embodiment isdescribed below with reference to FIG. 1A and FIG. 1B.

The projection system 11 includes, in the inside of the image correctionunit 5, a plurality of lookup tables (LUTs) that are storage areas (notshown) for storing gamma correction data. Each LUT corresponds to adisplay position on the screen for which gamma correction data isdetermined, that is each LUT corresponds to one display element of theimage display device. In the case in which the display screen is dividedinto a plurality of blocks each including a plurality of displayelements, each LUT corresponds to one block.

First, gamma correction data produced by the processing unit 30 of thePC 3 are transferred to LUTs of the image correction unit 5 via thecorrection data transfer board 33. A PC image signal is then transferredas an input signal to the image correction unit 5 from the graphic board31 of the PC 3. The image correction unit 5 performs the gammacorrection on the PC image signal given as the input signal, inaccordance with the gamma correction data stored in the LUTs. Asdescribed above, by the gamma correction, the PC image signal suppliedfrom the graphic board 31 is converted into a signal that will cause theluminance nonuniformity of the output signal of the projector 7 to becancelled out and will be outputted from the CCD camera 9. The resultantimage signal obtained via the gamma correction is output from the imagecorrection unit 5 to the liquid crystal projector 7. Thus, the liquidcrystal projector 7 projects an image with good luminance uniformityonto the screen 8.

(Another Example of the First Embodiment)

FIG. 8 is a flow chart showing another example of a process performed bythe image correction data calculator to calculate gamma correction dataused to correct an image including a plurality of primary colorcomponents, according to the first embodiment of the invention.

In the previous example of the first embodiment described above, theprojection system 11 projects an image having a single primary color. Incontrast, in this example of the first embodiment described below, imagedisplay means (for example, a liquid crystal projector having liquidcrystal panels responsible for three primary colors (RGB)) is capable ofdisplaying a color image including a plurality of primary colorcomponents. The method of producing gamma correction data described inthe previous example of the first embodiment can also be applied toproduction of gamma correction data for each color component R, G, or Bin the present example of the first embodiment.

Referring to FIG. 8, a process of calculating image correction data usedto correct an image having a plurality of primary color components isdescribed below. Note that similar reference numerals used in theprevious example according to the first embodiment described above withreference to FIG. 1A and FIG. 1B are used in the following description.

Under the control of the PC 3, the liquid crystal projector 7 projects atest image of each of three primary colors RGB onto the screen 8, andthe test image displayed on the screen 8 is captured by the CCD camera 9and resultant image data is input to the PC 3 in a similar manner as inthe previous example of the first embodiment. For a given color, theabove process is performed for each sampling point. After completion ofthe process for all sampling points, the process is performed for a nextcolor in a similar manner. Thus, a measured gamma characteristic isacquired for each of all primary colors (steps S201 to S205 in main stepS10).

Thereafter, as in the previous example of the first embodiment, the PC 3sets target gamma characteristics in main step S20 to the imagecorrection unit 5 (steps S206 and S207).

More specifically, in the setting of the target gamma characteristics ofR, G, and B signals, first, target images are set for a G input signal.That is, a target image for a maximum input signal level (“255”), and atarget image for a minimum input signal level (“0”), are set.Furthermore, a target gamma characteristic for intermediate input signalvalues between the minimum and maximum levels is set using an equationfor each display position.

Subsequently, for the R signal and then for the B signal, the targetimages set for the maximum input signal level (“255”) and for theminimum input signal level (“0”), respectively, used in the setting forthe G signal are set.

Thereafter, the processing unit 30 of the PC 3 finally calculates thegamma correction data in main step S30 on the basis of the target gammacharacteristic and the measured gamma characteristics (step S208).

In this example of the first embodiment, as described above, imagecorrection data, by which to achieve good RGB color balance at alldisplay positions and achieve a uniform color distribution over alldisplay positions, can be calculated.

(Second Embodiment)

FIG. 9 is a block diagram showing a projection system including of animage correction data calculator and a processing unit, according to asecond embodiment of the present invention.

As shown in FIG. 9, in this second embodiment of the present invention,an image correction data calculator 1A mainly includes: a PC 3 servingas processing means for executing an operating system and an imagecorrection data calculation program to implement an image correctiondata calculation method, an image splitter 13 for splitting the imagesignal output from the PC 3 into image signals of respective image areasallotted to the respective liquid crystal projectors 7 a and 7 b, whichwill be described later, a plurality of image correction units 5 a and 5b serving as image correction means for correcting the split imagesignals output from the image splitter 13 on the basis of the imagecorrection data supplied from the PC 3, a plurality of liquid crystalprojectors 7 a and 7 b responsible for respective three (RGB) primarycolors and serving as an image display device and image display meansfor projecting image data output from the respective image correctionunits 5 a and 5 b, a screen 8 serving as an image display device andimage display means for displaying the images projected by the liquidcrystal projectors 7 a and 7 b, and a CCD camera 9 serving as imagecapture means for capturing an image displayed on the screen 8.

The PC 3 includes, at least, an image input board 32 for inputting, tothe PC 3, a digital image signal output from the CCD camera 9, aprocessing unit 30 serving as analysis means for calculating imagecorrection data on the basis of the image signal input via the imageinput board 32 by executing the image correction data calculationprogram and also executing the operating system and other calculationprocessings, a correction data transfer board 33 for transferring gammacorrection data, which is image correction data calculated by theprocessing unit 30, to the image correction units 5 a and 5 b, and agraphic board 31 serving as image output means for outputting, under thecontrol of the processing unit 30, an RGB analog signal in a VGA formatto an image splitter 13.

The projection system 11A is formed by the PC 3, the image splitter 13,the image correction units 5 a and 5 b, the liquid crystal projectors 7a and 7 b, and the screen 8.

The details of the processing unit 30 are similar to those shown in FIG.1B.

An image correction data calculation method implemented by the imagecorrection data calculator 1A constructed in the above-described manneris described below based on FIG. 9 and referring to FIGS. 10 and 12.FIG. 10 is a flow chart showing the image correction data calculationmethod implemented by the image correction data calculator according tothe second embodiment of the present invention.

The image correction data calculator 1A produces gamma correction data,that is, image correction data by performing a process including threemain steps (S10 a, S20 a, and S30 a), as in the first embodiment.

In step S10 a, a gamma characteristic at each display element isacquired for each primary color and for each projector.

In step S20 a, a target gamma characteristic at each display element isset for each primary color and for each projector.

In step S30 a, gamma correction data for each display element iscalculated by the PC 3 serving as processing means, for each primarycolor and for each projector.

A process in step S10 a of acquiring the gamma characteristic at eachdisplay element for each primary color and for each of liquid crystalprojectors 7 a and 7 b is described below.

If input signal values of three RGB primary colors applied to the liquidcrystal projectors 7 a and 7 b are expressed as (R, G, B), samplingpoints for each primary color are given as follows.

That is, input signal values described below are selected as measurementsampling points for primary color R: (0, 0, 0), (16, 0, 0), (32, 0, 0),(48, 0, 0), (64, 0, 0), (80, 0, 0), (96, 0, 0), (112, 0, 0), (128, 0,0), (144, 0, 0), (160, 0, 0), (176, 0, 0), (192, 0, 0), (208, 0, 0),(224, 0, 0), (240, 0, 0), and (255, 0, 0).

For primary color G, input signal values described below are selected asmeasurement sampling points: (0, 0, 0), (0, 16, 0), (0, 32, 0), (0, 48,0), (0, 64, 0), (0, 80, 0), (0, 96, 0), (0, 112, 0), (0, 128, 0), (0,144, 0), (0, 160, 0), (0, 176, 0), (0, 192, 0), (0, 208, 0), (0, 224,0), (0, 240, 0), and (0, 255, 0).

For primary color B, input signal values described below are selected asmeasurement sampling points: (0, 0, 0), (0, 0, 16), (0, 0, 32), (0, 0,48), (0, 0, 64), (0, 0, 80), (0, 0, 96), (0, 0, 112), (0, 9, 128), (0,0, 144), (0, 0, 160), (0, 0, 176), (0, 0, 192), (0, 0, 208), (0, 0,224), (0, 0, 240), and (0, 0, 255).

For each of the measurement sampling points described above, a testimage is produced in advance for each of the liquid crystal projectors 7a and 7 b such that it has a signal level equal to the measurementsampling point at all display elements (or at predetermined displayelements), and stored in the PC 3.

An image of a test pattern is captured as follows. First, initial gammacorrection data that does not provide any correction effect at thisinitial stage are transferred from the PC 3 to respective imagecorrection units 5 a and 5 b corresponding to the liquid crystalprojectors 7 a and 7 b via the correction data transfer board 33.

Thereafter, a test image output from the PC 3 is split by the imagesplitter 13 and supplied to respective liquid crystal projectors 7 a and7 b such that the same test image is applied to all liquid crystalprojectors 7 a and 7 b. Thus, images are simultaneously displayed by allimage display devices in accordance with the input signal at apredetermined sampling point (step S301).

The test image displayed on the screen 8 is captured by the CCD camera9, and a resultant captured image is stored in the PC 3 via the imageinput board 32 of the PC 3 (step S302).

In the next step (S303), it is determined whether the process describedabove is completed for all sampling points. If the process is notcompleted, steps S301 to S303 are repeated, changing the sampling point,until the process is completed for all sampling points.

When it is determined in step S303 that the image has been captured forall sampling points, it is further determined (in step S304) whether theprocess is completed for all primary colors. If the process is notcompleted, steps S301 to S304 are repeated, changing the primary color,until the process is completed for all primary colors.

After completion of acquiring test images for all sampling points andfor all primary colors, the processing unit 30 of the PC 3 determinesthe gamma characteristics of predetermined display elements of each ofthe projectors 7 a and 7 b for each primary color, on the basis of thecaptured image stored in the PC 3 (steps S305 and S306).

Thereafter, from the gamma characteristics of the predetermined displayelements, the gamma characteristics of all display elements aredetermined for each primary color by means of four point interpolation,spline interpolation, or extrapolation, and resultant gammacharacteristics are stored in the PC 3 (steps S307 and S307 a).

Thereafter, a target gamma characteristic of each display element is set(calculated) of each of RGB colors in each of the liquid crystalprojectors 7 a and 7 b, in a similar manner as in the first embodimentdescribed above (steps S308 to S310 in main step S20 a).

In some multiprojection systems 11A, image display areas of the imagedisplay units, onto which images are projected by the respectiveprojectors 7 a and 7 b, do not overlap but are separated by physicalboundaries (such as joints or frames of the screen 8). In such amultiprojection system, when the target image is calculated simply bypassing the captured image signal through the lowpass filter, theluminance information of the physical boundaries, which are not parts ofthe image projected by the liquid crystal projectors 7 a and 7 b, exertsan influence on the displayed image. In this case, the target image isnot correct in areas close to the physical boundaries. To avoid theabove problem, in the calculation of the target image, it is necessaryto detect the physical boundary areas in the captured image and pass thecaptured image signal through the lowpass filter such that parts of thesignal corresponding to the physical boundary areas are not passedthrough the lowpass filter.

In some cases, there can be a slight difference in chromiticity ofprimary colors among the plurality of liquid crystal projectors 7 a and7 b. Such a difference causes a difference in output color even when theRGB ratios are adjusted to be equal in the above-described manner. Sucha difference can be deleted by setting separately the different RGBratios for the respective liquid crystal projectors 7 a and 7 b suchthat white obtained by additively mixing primary colors becomes equal inchromaticity for all liquid crystal projectors 7 a and 7 b, and thensetting target gamma characteristics on the basis of the RGB ratiosdetermined in the above-described manner, and finally calculating theimage correction data. Use of the image correction data obtained in theabove-described manner allows elimination of color differences amongimage display areas.

By taking the image of each of the RGB primary colors using the CCDcamera 9 attached with a white balance adjustment filter, it is possibleto determine the RGB ratio for each projector such that the colordifferences among the image display areas are minimized. If the whitebalance adjustment filter has a characteristic equivalent or similar toluminous efficacy of a human eye, high accuracy can be achieved in thewhite balance adjustment.

In the next step S30 a, the process of determining the gamma correctiondata at each display element of each RGB primary color in each of theplurality of liquid crystal projector 7 a and 7 b is performed inbasically the same manner as in the first embodiment described above(step S311). That is, the processing unit 30 of the PC 3 calculates thegamma correction data for each display element of each of the liquidcrystal projectors 7 a and 7 b such that the measured gamma curvebecomes equal to the target gamma curve.

(Use of Monochrome Camera in the Second Embodiment)

In the second embodiment described above, the color CCD camera 9 isused. Instead of the color CCD camera 9, a monochrome CCD camera may beused in implementation of the image correction data calculation method.In this case, the process is performed as shown in FIG. 11. FIG. 11 is aflow chart showing a process of acquiring measured gamma characteristicsusing different filters for respective RGB colors, in the imagecorrection data calculator according to the second embodiment of thepresent invention.

Displayed images can be captured accurately and clearly, if a monochromeCCD camera 9 attached with a color filter corresponding to one ofprimary colors is used, and the image is captured a plurality of timessuch that a different color filter corresponding to a different primarycolor is used each time the image is captured. By using color filtersthat can separate RGB primary colors from each other, it is possible tomore accurately acquire the gamma characteristics for each of RGBprimary colors.

The process is described in further detail below.

First, a predetermined color (one of RGB primary colors with apredetermined gray level) is displayed on the screen (step S601), and acolor filter corresponding to the displayed primary color is used (stepS602).

An image of the displayed primary color is captured by the CCD camera 9via the selected color filter, and resultant image data is stored in thePC 3 (step S603).

The processing unit 30 of the PC 3 determines whether capturing of theimage is completed (for all gray levels and for all RGB colors) (stepS604). If the capturing of the image is not completed, steps 601 to S604described above are repeated. If the capturing of the image iscompleted, gamma characteristic data is calculated on the basis of theacquired images (step S605).

The process after acquisition of the measured gamma characteristic datais performed in a similar manner as in steps S20 a and S30 a describedabove with reference to FIG. 10.

(Use of White Balance Filter in the Second Embodiment)

FIG. 12 is a flow chart showing a process of acquiring white balancedata and measured gamma characteristics by using a white balanceadjustment filter, in the image correction data calculator according tothe second embodiment of the present invention.

In FIG. 12, the PC 3 supplies a predetermined color (one of RGB primarycolors with a predetermined gray level) to the liquid crystal projectors7 a and 7 b thereby displaying the predetermined color on the screen(step S701). The PC 3 specifies a filter corresponding to the displayedprimary color to be used (step S702). An image of the displayed primarycolor is captured by the CCD camera 9 via the specified filter, andresultant image data is stored in the PC 3 (step S703).

The PC 3 then determines whether the currently displayed image is awhite balance (WB) measurement image (an RGB primary color imagecorresponding to a maximum input level or an input level of 0) (stepS704). If a WB measurement image is currently displayed, the PC 3 issuesa command to use a WB filter (step S705). Then, an image of thedisplayed primary color is captured by the CCD camera 9 via thespecified filter, and resultant image data is stored in the PC 3 (stepS706).

The PC 3 determines whether the capturing of the image using the WBfilter is completed (step S707). If the capturing of the image is notcompleted, steps S705 to S707 described above are repeated.

If it is determined in step S707 that the capturing of the image usingthe WB filter is completed, or if it is determined in step S704 that thecurrently displayed image is not a WB measurement image, then it isdetermined whether capturing of the image of the predetermined color iscompleted (for all RGB primary colors and for all gray levels) (stepS708). If the capturing of the image is not completed, steps S701 toS708 described above are repeated.

If it is determined in step S708 that the capturing of the image iscompleted, the PC 3 calculates the white balance adjustment data and thegamma characteristic data from the acquired image data (step S709).

The image correction data calculator according to the second embodimentoperates in the above-described manner to acquire necessary gammacorrection data.

In the second embodiment described above, the gamma correction data isdetermined for each display element. Alternatively, in order to performthe process in a shorter time using a less memory capacity, the displayscreen area may be divided into blocks each including, for example, 4×4display elements, and the gamma correction data may be determined foreach block such that the same gamma correction data is applied to alldisplay elements in each block. In this case, it is desirable that theblock size be varied adaptively depending on the degree of nonuniformityof luminance. That is, a small block size is used for an area whereluminance nonuniformity is large, while a large block size is used foran area where luminance nonuniformity is small. In particular, when themultiprojection system is of the seamless type, it is desirable that thegamma correction data be determined for each display element inoverlapping areas but, in the other areas, gamma correction data bedetermined for each block. This makes it possible to achieve highcorrection accuracy and thus output a high-uniformity image in a shorttime using a low-capacity memory.

Alternatively, only the gamma correction data for predetermined displayelements may be calculated on the basis of measured values, and thegamma correction data for the other display elements may be determinedby means of four-point interpolation, spline interpolation, orextrapolation and by using the gamma correction data which is marginallylocated to display elements of which interpolation data is alreadycalculated. This allows reductions in the measurement time and theprocessing time needed to calculate the gamma correction data.

(Projection System Using Image Correction Data According to the SecondEmbodiment)

The projection system 11A is capable of correcting an image using imagecorrection data produced by the image correction data calculation methodaccording to the second embodiment.

The image correction units 5 a and 5 b include a plurality of lookuptables (LUTs) for storing gamma correction data. One LUT is produced fora predetermined display position on the screen for each RGB primarycolor. Note that the expression “predetermined display position” is usedto describe a display position at which the gamma correction data isdetermined, and thus the “predetermined display position” may be each ofall display positions or the position of each block including aplurality of display elements.

First, the gamma correction data produced by processing unit 30 of thePC 3 for each display element of each RGB primary color, in each of theliquid crystal projectors 7 a and 7 b is transferred from the correctiondata transfer board 33 of the PC 3 to a corresponding LUT of the imagecorrection units 5 a and 5 b.

Subsequently, a PC image signal is supplied as an input signal to theimage splitter 13 from the graphic board 31 of the PC 3.

The image splitter 13 splits the given input signal into as many imagesignals as the number of liquid crystal projectors 7 a and 7 b, andsupplies the resultant respective image signals to the correspondingimage correction units 5 a and 5 b.

The image correction units 5 a and 5 b obtain gamma-corrected imagesignals by applying the gamma correction data to the input signals. Theobtained image signals are supplied to the liquid crystal projectors 7 aand 7 b, which in turn project images onto the screen 8 in accordancewith the received image signals.

In the above process, the PC image signal is converted to the outputimage data, using gamma correction data stored in LUTs corresponding todisplay positions on the screen for respective RGB primary colors in theimage correction units 5 a and 5 b, such that nonuniformity of luminanceat outputs of the projectors is eliminated for any input signal level ofany RGB primary color, and thus the resultant displayed image has gooduniformity in luminance and color.

In the second embodiment described above, images are captured using thecamera to acquire measured gamma characteristics. However, when an imageprojected by a plurality of projectors is captured by a camera, if thereis a large luminance difference among projectors, flaring can causemeasured values to become incorrect in some area. This problem can besolved by a technique described below in a third embodiment.

(Third Embodiment)

FIG. 13 is a flow chart showing a process of calculating optimum gammacorrection data according to a third embodiment of an image correctiondata calculation method of the present invention. A image correctiondata calculator used in this third embodiment is the same in terms ofstructure as that shown in FIG. 9, and thus FIG. 9 is referred to, asrequired, in the following description.

At the beginning of the process, it is determined whether there is gammacorrection data (step S501). Because there is no gamma correction dataat this initial stage, gamma correction data is calculated in a similarmanner as in the second embodiment described above. That is, an inputsignal with a predetermined signal level is displayed on the display(step S502), and gamma correction data for each display element iscalculated (step S503). Thereafter, the process returns to step S501.

At this time, it is determined in step S501 that there is acquired gammacorrection data. Thus, the input signal with the predetermined level iscorrected using the gamma correction data and displayed on the display(step S504). The display image is then captured into the PC 3 via theCCD camera 9, and measured gamma characteristics and target gammacharacteristics are calculated (step S505).

Subsequently, it is determined whether the measured gammacharacteristics are nearly the same as the target gamma characteristics(more specifically, for example, it is determined whether thedifferences between the measured characteristics and the target gammacharacteristics are within a predetermined allowable range) (step S506).If they are not nearly the same, the gamma correction data isre-calculated for each display element on the basis of the measuredgamma characteristics, the target gamma characteristics, and the gammacorrection data determined in previous steps (step S507), and theprocess returns to step S501.

The above-described process is performed repeatedly until it isdetermined in step S506 that the measured gamma characteristics arenearly the same as the target gamma characteristics. If it is determinedin step S506 that the measured gamma characteristics are nearly the sameas the target gamma characteristics, the process is ended (step S508).

When the process including the step of capturing the image using thecamera to acquire the measured gamma characteristic is iterated, thedifference in luminance among the plurality of projectors graduallydecreases, and finally very accurate image correction data can beobtained without being influenced significantly by flaring or the likeof the camera.

In the third embodiment described above, the image correction data isre-calculated until the measured gamma characteristics become nearly thesame as the target gamma characteristics. Alternatively, there-calculation may be terminated when the number of iterations reaches apredetermined upper limit.

FIG. 14 is a flow chart showing another example of a process ofacquiring measured gamma characteristics based on the image correctiondata calculation method according to the third embodiment of the presentinvention.

In this example, as shown in FIG. 14, when the PC 3 acquires themeasured gamma characteristics, the projectors are first adjusted suchthat the luminance difference is reduced to a certain degree (stepS401), and then steps described in a block enclosed in a broken line, B,in FIG. 10 are performed to acquire the measured gamma characteristics(step S402).

Even in this method in FIG. 14, the same effect as the one explained inthe third embodiment, in which calculations are repeated by feeding backthe previous image correction data, is obtained.

An embodiment of a projection system capable of correcting an imageusing image correction data obtained in the above-described manner isdescribed below.

(Fourth Embodiment)

FIG. 15 is a block diagram showing an image correction unit used in aprojection system according to a fourth embodiment of the presentinvention.

An input signal includes a synchronization signal and image data.

The image correction unit 5 includes an address counter 51 for countingthe synchronization signal thereby calculating a pixel address (addressof a display element) and outputting the calculated address, a pluralityof lookup tables LUT1 to LUTn for storing in advance image correctiondata and outputting corrected image data produced in response toreceiving image data, a switching circuit 52 serving as a switching unitfor selecting one of lookup tables LUT1 to LUTn depending on the pixeladdress received from the address counter 51 and outputting datasupplied from the selected lookup table, and a video signal converter 53serving as video signal conversion means for converting the output fromthe switching circuit 52 into a video signal adapted to the liquidcrystal projector 7.

The plurality of lookup tables LUT1 to LUTn are produced such that thereis one lookup table corresponding to each of display elements formingthe output screen.

The image correction unit 5 operates as follows.

As described above, the synchronization signal of the input image signalis input to the address counter 51 of the image correction unit 5, andthe address counter 51 outputs a pixel address determined on the basisof the synchronization signal.

The image data of the input image signal is simultaneously input to theplurality of lookup tables LUT1 to LUTn whose outputs are connected tothe switching circuit 52.

The switching circuit 52 selects one of lookup tables LUT1 to LUTncorresponding to the pixel address output from the address counter 51and outputs (corrected) image data supplied from the selected lookuptable to the video signal converter 53.

The video signal converter 53 converts the image data received from theswitching circuit 52 into an analog video signal and outputs theresultant analog video signal to the liquid crystal projector.

(Fifth Embodiment)

FIG. 16 is a block diagram showing an image correction unit used in aprojection system according to a fifth embodiment of the presentinvention.

As shown in FIG. 16, the image correction unit 5A includes an addresscounter 51 for counting a synchronization signal of an input imagesignal thereby calculating a pixel address and outputting the calculatedaddress, a plurality of lookup tables LUT1 to LUTn provided forrespective RGB primary colors, for storing in advance image correctiondata associated with respective RGB primary colors and for producing andoutputting corrected image data in response to receiving the respectiveRGB color components of input image data, switching circuits 52R, 52G,and 52B provided for respective RGB primary colors, each serving toselect one of lookup tables LUT1 to LUTn depending on the pixel addressoutput from the address counter 51 and output data supplied from theselected lookup table, and an RGB video signal converter 53 forconverting the image data output from the respective switching circuits52R, 52G, and 52B into RGB video signals adapted to the liquid crystalprojector 7.

In this image correction unit 5A, by operating in parallel the lookuptables LUT1 to LUTn provided for respective RGB primary colors and theswitching circuits 52R, 52G, and 52B provided for respective RGB primarycolors, it is possible to output an image with corrected luminance andcolor uniformity.

In the specific example described above, lookup tables LUT1 to LUTn areprovided for respective display elements. Alternatively, lookup tablesmay be provided for respective blocks each including a plurality ofdisplay elements. This allows a large reduction in memory space neededto store lookup tables LUT1 to LUTn, and thus a reduction in productioncost of the apparatus can be achieved.

In the specific example described above, the switching circuits 52R,52G, and 52B are connected to the respective lookup tables LUT1 to LUTnsuch that a necessary lookup table is selected from the lookup tablesLUT1 to LUTn and only data provided by the selected lookup table isoutput via the switching circuits 52R, 52G, and 52B. Alternatively, theplurality of lookup tables LUT1 to LUTn may be controlled such that onlynecessary data is output from one of the lookup tables LUT1 to LUTn tothe switching circuits 52R, 52G, and 52B.

(Sixth Embodiment)

Referring to FIGS. 17 and 18, a sixth embodiment of the presentinvention is described below. FIG. 17 is a block diagram showing animage correction unit used in a projection system according to a sixthembodiment of the present invention. FIG. 18 is a block diagram showingan arrangement of lookup tables LUT1 to LUTn in an image correction unitof a projection system according to the sixth embodiment of the presentinvention.

As shown in FIG. 17, the image correction unit 5B includes an addresscounter 51 for counting a synchronization signal of an input imagesignal thereby calculating a pixel address and outputting the calculatedaddress, a plurality of lookup tables LUT1 to LUTn for storing inadvance image correction data and outputting corrected image datacreated in response to reception of image data of an input image signal,a switching circuit 52 a serving as a selection unit for selecting oneof lookup tables LUT1 to LUTn depending on the pixel address receivedfrom the address counter 51 and outputting data supplied from theselected lookup table, and an interpolation coefficient generation unit55 serving as a coefficient generation unit for generating aninterpolation coefficient depending on the pixel address received fromthe address counter 51, a plurality of (four, in this specific example)multipliers 541 to 544 serving as coefficient operation units andmultipliers for multiplying the output of the selection circuit 52 a bythe interpolation coefficient generated by the interpolation coefficientgeneration unit 55, an adder 56 for adding together the outputs of themultipliers 541 to 544 thereby producing and outputting corrected imagedata, and a video signal converter 53 for converting the output of theadder 56 into a video signal adapted to the image display device.

The plurality of lookup tables LUT1 to LUTn are provided such that thereis one lookup table for each of areas defined on the output screen. Inthis specific embodiment, 49 areas are defined on the output screen, at7×7 lattice points as represented by open circles in FIG. 18.

Image data of an input image signal is simultaneously input to alllookup tables LUT1 to LUTn. The outputs of the lookup tables LUT1 toLUTn are connected to the selection circuit 52 a.

The selection circuit 52 a selects outputs of four LUTs corresponding tolocations surrounding a display element specified by a pixel addressoutput from the address counter 51, and supplies the selected outputs torespective four multipliers 541 to 544. For example, if a displayelement at a location denoted by a solid circuit in FIG. 18 is specifiedby a pixel address output from the address counter 51, the selectioncircuit 52 a selects LUTs corresponding to locations a, b, c, and dsurrounding the above display element and supplies the outputs of theLUTs to the respective four multipliers 541 to 544.

The interpolation coefficient generation unit 55 generates interpolationcoefficients according to the pixel address output from the addresscounter 51. In the specific example shown in FIG. 18, the interpolationcoefficients are used to interpolate the image data for the displayelement at the location indicated by the solid circle based on the imagedata at the locations a, b, c, and d. The interpolation coefficients maybe normalized. The multipliers 541 to 544 multiply the outputs of theselected LUTs by the respective interpolation coefficients.

After the image data are multiplied by the interpolation coefficients bythe multipliers 541 to 544, the resultant image data are added togetherby the adder 56 and converted into a video signal by the video signalconverter 53. The resultant video signal is supplied to the liquidcrystal projectors 7 a and 7 b. In accordance with the received videosignal, the liquid crystal projectors 7 a and 7 b project an image onthe screen 8.

By performing the process in the above-described manner, imagecorrection data associated with each display element specified by apixel address is interpolated and calculated with respect to thelocation of the display element by means of interpolation using imagecorrection data supplied from LUTs.

In the case of a color image, circuits similar to that described aboveare provided for respective RGB primary colors, and the process isperformed in parallel by the circuits. The resultant image output viathe process described above has luminance and color uniformitycorrected.

In the specific example described above, LUTs are provided correspondingto respective 49 lattice points wherein the point-to-point distance isset to be greater in a central area and smaller in an peripheral area ofthe screen. It means, by setting the point-to-point distance to besmaller in an area where the measured gamma characteristic variesgreatly, it becomes possible to efficiently reduce correction errors.

Although in the sixth embodiment described above, LUTs are set atrespective 49 lattice points, the number of lattice points is notlimited to 49. For example, LUTs may be set at respective 25 points (5×5lattice points). This results in a large reduction in memory spaceneeded to store lookup tables LUTs, and thus a reduction in productioncost of the apparatus can be achieved. Of course, the number of latticesis not limited to these numbers.

(Seventh Embodiment)

Referring to FIGS. 19 and 20, a seventh embodiment of the presentinvention is described below. FIG. 19 is a block diagram showing anotherimage correction unit used in a projection system according to a seventhembodiment of the present invention.

The image correction unit 5C includes an address counter 51 for countinga synchronization signal of an input image signal thereby calculating apixel address and outputting the calculated address, a plurality oflookup tables LUT1 to LUTn, in which image correction data are stored inadvance and output corrected image data produced in response toreceiving image data of an input image signal, an LUT number table 57for outputting a LUT number corresponding to a pixel address receivedfrom the address counter 51, a selection circuit 52 a for selecting anLUT in accordance with the LUT number output from the LUT number table57 and outputting image data supplied from the selected LUT, and a videosignal converter 53 for converting the output of the selection circuit52 a into a video signal adapted to the image display device.

There are prepared as many lookup tables LUT1 to LUTn as predetermined.Herein, it is assumed that there are prepared 256 lookup tables LUT 1 toLUTn. That is, it is assumed herein that n=256. The LUT number table 57stores data indicating which one of 256 lookup tables LUT1 to LUTnshould be selected for a given pixel address.

The operation the image correction unit 5C is described below withreference to FIG. 20. FIG. 20 is a flow chart showing a process ofcalculating data to be stored in LUTs disposed in the image correctionunit of the projection system according to the seventh embodiment of theinvention.

First, image correction data is calculated for each of all displayelements in a similar manner as described above with reference to thesecond embodiment (step S801). When the image correction data isdetermined for each of all display elements, if an output image is, forexample, in the VGA format, as many LUT data as 640×480=307200 arecalculated. When image correction data is determined for each block, thenumber of LUT data becomes smaller.

The LUT data are then subjected to principal component analysis andclustered into a predetermined number of groups. Via this process, forexample, 256 representative LUT data are obtained, and an LUT number isassigned to each representative LUT data (step S802). The 256representative LUT data are stored as table data in the respective 256LUTs described above.

Then each of the 307200 LUT data is examined to determine which one ofthe clustered 256 LUTs each LUT data is closest to, and thecorresponding LUT number is calculated (step S803).

The 256 LUT data determined in the above-described manner are thentransferred to the corresponding lookup tables LUT1 to LUTn (step S804).

Subsequently, data indicating the LUT number of 307200 LUTs istransferred to the LUT number table 57 and LUT numbers are written incorresponding locations of the LUT number table 57 (step S805).

Thus, data written in LUTs and data written in the LUT number table arecalculated.

A process of correcting an image is described below.

Image data of an input image signal is simultaneously input to alllookup tables LUT1 to LUTn. The outputs of the lookup tables LUT1 toLUTn are connected to the selection circuit 52 a.

On the other hand, the synchronization signal of the input image signalis input to the address counter 51, and the address counter 51 outputs apixel address determined on the basis of the synchronization signal. Theresultant pixel address is input to the LUT number table 57. The LUTnumber table 57 outputs an LUT number corresponding to the receivedpixel address.

The selection circuit 52 a selects an LUT corresponding to the LUTnumber received from the LUT number table 57, and the selection circuit52 a supplies the output of the selected LUT to the video signalconverter 53.

In the case of a color image, circuits similar to that described aboveare provided for respective RGB primary colors, and the process isperformed in parallel by the circuits. The resultant image output viathe process described above has corrected luminance and coloruniformity.

Although in the seventh embodiment described above, as many as 256 LUTsare used, a similar correction can be achieved using a less number ofLUTs if LUT data are normalized, as described below in a firstmodification of the seventh embodiment.

(First Modification of the Seventh Embodiment)

FIG. 21 is a block diagram showing a first modification of the imagecorrection unit used in a projection system, according to the seventhembodiment of the present invention.

In this first modification, the image correction unit 5D includes, inaddition to the compartments of the image correction unit 5C shown inFIG. 19, a coefficient memory 59 connected to the output of the addresscounter 51, for storing coefficients (ratios of original values tonormalized values) depending on pixel addresses, and a multiplier 58connected to the output of the selection circuit 52 a, for multiplyingthe output of the selection circuit 52 a by a coefficient output fromthe coefficient memory 59.

If handling of offset components of LUTs is performed in the outside ofthe LUTs, it becomes possible to achieve a similar correction effectusing a still less number of LUTs, as described below in a secondmodification.

(Second Modification of the Seventh Embodiment)

FIG. 22 is a block diagram showing a second modification of the imagecorrection unit used in a projection system, according to the seventhembodiment of the present invention.

In this second modification, the image correction unit 5E includes, inaddition to the components of the image correction unit 5D shown in FIG.22, an offset memory 61 serving as a constant memory connected to theoutput of the address counter 51, for storing a constant (offset), andan adder 60 connected to the output of the multiplier 58, for adding theconstant (offset) output from the offset memory 61 to the output of themultiplier 58.

As described above, the offset memory 61 stores the constant indicatingthe difference between original values and values obtained after theoffset is removed. Although the offset memory 61 stores only oneconstant in the specific example described above, the offset memory 61may store a plurality of constants corresponding to particular positionson the screen.

The multiplier 58 or the adder 60 is not necessarily needed to belocated at the stage following the selection circuit 52 a, but may bedisposed at the stage before the LUTs.

In the seventh embodiment described above, LUTs are calculated for alldisplay elements. Alternatively, LUTs may be calculated for respectiveblocks each including a plurality of display elements. This results in alarge reduction in memory space needed to store the LUT number table 57,and thus a reduction in production cost of the apparatus can beachieved.

As for each projector used in the embodiments described above, atransmissive liquid crystal projector, a reflective liquid crystalprojector, a DLP projector using a digital micromirror device (DMD), ora similar projector may be employed.

The present invention is not limited to the details of the embodimentsdescribed above, but various modifications or applications are possiblewithout departing from the spirit and scope of the invention.

INDUSTRIAL APPLICABILITY

As described above, the method and apparatus for calculating imagecorrection data according to the present invention makes it possible toautomatically and easily produce image correction data for use inreducing at least one of luminance nonuniformity and color nonuniformityover the entire input image displayed on the image display device.

The projection system according to the present invention is capable ofdisplaying an image whose luminance nonuniformity and/or colornonuniformity are reduced over the entire input image displayed on theimage display device, by using image correction data produced by theimage correction data production method or the image correction dataproduction apparatus.

1. A method of calculating image correction data comprising: acquiring,based on image data captured by an image capture section, aninput-output characteristic at each of a plurality of display elementson a display screen of an image display section that includes at leastone image display unit; setting a target input-output characteristic tobe obtained at each of the plurality of display elements by calculatingthe target input-output characteristic based on data of the acquiredinput-output characteristic; and calculating image correction data tocorrect an input-output characteristic for an input image signal inaccordance with locations of display elements on the screen, based onthe acquired input-output characteristic and the target input-outputcharacteristic for each of the display elements.
 2. The method ofcalculating image correction data, according to claim 1, whereincalculating the target input-output characteristic comprises setting amaximum value and a minimum value of the target input-outputcharacteristic to be obtained, and setting the target input-outputcharacteristic between the minimum and maximum values to be an averagevalue of the acquired input-output characteristics at positions over thescreen.
 3. The method of calculating image correction data, according toclaim 2, wherein the maximum value of the target input-outputcharacteristic is set such that a luminance distribution correspondingthereto is equal to a luminance distribution that is obtained when imagedata obtained by capturing an image display corresponded to apredetermined input signal at respective positions on the screen ispassed through a lowpass filter.
 4. The method of calculating imagecorrection data, according to claim 2, wherein the minimum value of thetarget input-output characteristic is set such that a luminancedistribution corresponding thereto is equal to a luminance distributionthat is obtained when image data obtained by capturing an image displaycorresponded to a predetermined input signal at respective positions onthe screen is passed through a lowpass filter.
 5. A projection systemcomprising: image output means for outputting image data to bedisplayed; image correction means for correcting the image data outputfrom the image output means in accordance with image correction data;and image display means for displaying on a screen the image datacorrected by the image correction means; wherein the image correctionmeans includes: a plurality of lookup tables which store imagecorrection data; a lookup table number table for storing lookup tablenumbers in relation to corresponding positions on the screen; aselection unit for selecting an effective lookup table from theplurality of lookup tables in accordance with a value output from thelookup table number table and for outputting corrected image data thathas been corrected based on the image correction data stored in theselected effective lookup table; and video signal output means forconverting the corrected image data from the selection unit into a videosignal adapted to the image display device; wherein the plurality oflookup tables comprise a plurality of conversion tables that reflect aresult of statistical processing on a plurality of image correction dataover the screen of the image display means; and wherein the lookup tablenumber table stores information indicating one of the conversion tablesthat best approximates image correction data at a given position on thescreen for each of said corresponding positions on the screen; andwherein the conversion table for the given position is determined bycomparing the image correction data calculated at the given positionwith respective conversion tables stored in the lookup tables.